Nato says it has witnessed an "alarming" build-up of Russian military forces near the border with Ukraine, as Kiev described a Russian aid convoy entering the country as a "direct invasion".

Nato Secretary General Anders Fogh Rasmussen said the increase in ground and air forces just miles from where the Ukrainian military is battling pro-Moscow separatists could lead to the further isolation of Russia.

"We have also seen transfers of large quantities of advanced weapons, including tanks, armoured personnel carriers and artillery to separatist groups in eastern Ukraine," he said in a statement.

Image Caption:Ukraine insisted the aid convoy entered under the auspices of the Red Cross

More than 100 trucks from a Russian aid convoy crossed into the country without permission on Friday and have begun arriving in the eastern city of Luhansk.

It had been agreed the lorries would only be allowed into eastern Ukraine if they were escorted by the International Red Cross.

But the charity has pulled out after not receiving enough security guarantees as fighting continues to rage.

Kiev's foreign ministry said it had informally allowed the convoy to pass to avoid "provocations" and state security chief Valentyn Nalivaychenko said his country will not use force against it.

But he also said: "We consider this a direct invasion by Russia of Ukraine. Under the cynical guise of the Red Cross these are military vehicles."

A spokesman for the European Union foreign affairs chief Baroness Ashton criticised Russia for a "clear violation" and urged Moscow to reverse its decision.

The Pentagon said it was "very concerned" and called for the convoy to be pulled out immediately.

The vehicles are carrying water, generators and sleeping bags reportedly intended to help civilians in Luhansk, where pro-Russian separatist fighters are besieged by Ukrainian government forces.

The trucks, part of a 280-lorry convoy, had been held at a border crossing for a week amid suspicions by Kiev that the mission was being used as a cover for an invasion by Moscow.

Ukraine has been reluctant to let the convoy through over fears it is carrying supplies for the rebels, despite Moscow's insistence the goods were simply humanitarian aid.

Russian President Vladimir Putin told Germany's Angela Merkel that any further delays to the aid would be unacceptable.

The Kremlin said Russia decided to go ahead with the mission after "clear procrastination" by the Kiev government.

The transit comes as the Ukrainian president said he would tell Mr Putin to rein in pro-Moscow separatists, when the two leaders meet next week.

Petro Poroshenko said he would tell President Putin that Ukraine had "a strong country, a strong army" behind him.

He said: "We are capable of defending our sovereignty, our independence and our territorial integrity - we are fighting for the independence of Ukraine. Together we will win for sure."